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What external beam radiotherapy is

External radiotherapy destroys cancer cells using radiation aimed at a cancer from a machine. The most common types of external radiotherapy use high energy X-ray beams, such as photon beams. Other types of radiation used include particle beams, such as protons or electrons. The radiotherapy beams destroy the cancer cells in the treatment area.

You have treatment in the radiotherapy department from equipment similar to a large X-ray machine. Radiotherapy machines vary slightly in how they look and how they work. The most common type of machine is called a linear accelerator machine (LINAC), which uses electricity to create the radiotherapy beams.

Below is a 360° photograph of a radiotherapy room containing a linear accelerator machine. Use the arrows to move the picture and look around the room.

The radiotherapy machine itself does not touch you and you don’t feel any pain during the treatment. But you may get some discomfort or pain later on from the side effects. Before starting your treatment your doctor will discuss with you the possible short term and long term side effects of your treatment. Most side effects are temporary and can usually be well controlled with medicines.

Your treatment team uses images from a range of tests, which may include X-rays, CT scans, MRI scans, or PET scans, to plan and monitor your treatment. They plan the radiotherapy to give a high dose to the cancer. It is important that the radiotherapy field covers the whole cancer, plus a border around it. This helps the radiotherapy to work as well as possible in treating the cancer. Doctors try to give as low a dose as possible to the surrounding healthy tissue to reduce the risk of side effects.

We have information about planning radiotherapy treatment.


How radiotherapy works

Radiotherapy treatment works by damaging the DNA within the cancer cells. DNA is the genetic code that controls how the body's cells behave. The radiotherapy either damages the DNA directly or creates charged particles called free radicals that damage the DNA.

Cancer cells stop growing or die when their DNA is damaged. When the cells die the body breaks them down and gets rid of the waste substances. Normal cells in the radiotherapy area may also be damaged but they can usually repair themselves.


Radiotherapy fractions

Radiotherapy treatment to destroy and possibly cure a cancer is called radical radiotherapy or curative radiotherapy. The full dose of radiation is usually divided into a number of smaller doses called fractions. This allows healthy cells to recover between treatments. You have the fractions as a series of treatment sessions which make up your radiotherapy course.

Radiotherapy to relieve cancer symptoms, such as pain, is called palliative radiotherapy. You often have this in fewer fractions and sometimes it is just one treatment. The radiotherapy dose with each fraction is bigger but you have fewer fractions so the overall radiotherapy dose is lower. Palliative radiotherapy has fewer side effects than curative radiotherapy.


Your course of treatment

When deciding on your course of treatment your radiotherapy doctor takes into account

  • Your type of cancer
  • The position of the cancer in the body
  • Any other treatment you've had, are having, or is planned for you
  • Your general health and fitness

This means that your treatment will be individual to you.

A course of radiotherapy aimed at curing cancer usually lasts between 1 to 6 weeks but may be longer than this. Your doctor will tell you how long your treatment will take.

Many people have daily treatment from Monday to Friday, with a rest at weekends. Others have different treatment plans, such as 3 days a week, or treatment twice a day, including at weekends. The radiotherapy staff try to give you appointments for the same time each day so that you can get into a routine.

Your doctor chooses the type of radiotherapy and the machine for your treatment according to the type of cancer you have and where it is in your body. The different types of treatment are described below.


Conformal radiotherapy

Conformal radiotherapy is a common type of external beam radiotherapy. It is also called 3D conformal radiotherapy (3D CRT). It uses computers and radiotherapy machines to shape the high dose of radiotherapy closely around the tumour. It aims to give a high dose of radiation to the cancer while reducing radiation damage to healthy body tissues and organs.

We have detailed information about conformal radiotherapy.


Intensity modulated radiotherapy (IMRT)

Intensity modulated radiation therapy can shape the radiotherapy very closely around the tumour. The radiotherapy machine aims the beams at the tumour from many different directions. Spreading the dose out helps to lower the dose of radiation to normal healthy cells nearby.

IMRT can also vary the strength of the beams across the tumour during each dose of treatment. So the machine gives very precise doses to the cancer or to specific areas within the tumour. 

IMRT can create a U shaped (concave) area at the edge of the radiotherapy field to avoid high radiation doses to structures that would be damaged by the radiotherapy. This can reduce the risk of long term side effects. This is very helpful in areas such as the head and neck, for example to avoid the spinal cord or salivary glands.

The treatment team carefully plans the treatment using 3D CT scan images. They also use computerised dose calculations to find the dose strength pattern that best matches the tumour shape. The planning may take longer than for conformal radiotherapy. Many radiotherapy machines also have built in scanners (imaging units) so that they combine IMRT and image guided radiotherapy

IMRT is currently used to treat many different types of cancer. We have detailed information about IMRT.


Image guided radiotherapy (IGRT)

Image guided radiotherapy can help the treatment team to target the radiotherapy treatment more accurately for some types of cancer. It may reduce the dose of radiation to normal tissues and so may help to reduce side effects. 

With IGRT you may have X-rays and scans (images) immediately before or during a radiotherapy treatment session. Some people have small markers put into the area of the tumour. The markers show up on X-ray and allow the radiographer to target the tumour accurately.

4-dimensional radiotherapy (4D-RT)

Some tumours may be in a different place for each treatment. An example is the prostate gland, which can move depending on whether your bladder is full or not. The radiographers can use image guided radiotherapy to take scans before each treatment. Then they can make sure the cancer is inside the radiotherapy field. Because this treatment uses 3 dimensional planning plus the 4th dimension of time, it is called 4D radiotherapy.

Some radiotherapy machines can take scans or X-rays during the treatment session. This type of treatment is helpful for cancers in areas of the body that move when we breathe, such as the lung. Computers process the scans to show changes in the tumour position. Your radiographer can then change your position during treatment or adjust the radiotherapy dose. Some machines can switch off the radiation if the tumour moves out of the radiotherapy field and switch it back on when the tumour moves back into the field. 

We have a page of information about image guided radiotherapy.


Stereotactic radiotherapy and radiosurgery

You have stereotactic radiotherapy (SRT) in one session or a small number of sessions. These treatments are mainly available at specialist cancer centres. Stereotactic treatments may also be called radiosurgery. 

These treatments use smaller radiation fields and higher doses for each treatment (fraction) than other types of external beam radiotherapy. You can have this type of treatment using the usual type of radiotherapy machine (the linear accelerator) that has been specially designed. Or you may have it using a specific machine called CyberKnife or Gamma knife. The Gamma Knife machine can only treat brain tumours.

Stereotactic radiotherapy directs beams from more angles than other types of radiotherapy. So the treatment can target the tumour very precisely. 

Each of the individual beams gives a small dose of radiotherapy. Because all the beams meet at the tumour, the cancer gets the full dose. The radiotherapy team plans the treatment so that each beam is aimed very precisely. This allows them to give a very high dose of radiation to the tumour but a much lower dose to the surrounding normal tissues.

Stereotactic radiotherapy for brain tumours

Doctors can use stereotactic radiotherapy for small, well defined brain tumours, such as acoustic neuromas, pituitary adenomas, haemangioblastomas or secondary brain tumours. This is called stereotactic cranial radiotherapy or stereotactic radiosurgery. This type of radiotherapy can also treat spinal cord tumours. During treatment for a brain tumour you may have a head frame or mask to hold your head still.

We have detailed

Stereotactic body radiotherapy (SABR)

Doctors may also use stereotactic radiotherapy for small, well defined tumours that are deep within the body. They are most likely to use it for tumours in the lung, liver or pancreas. This type of radiotherapy may be called stereotactic body radiotherapy (SBRT) or stereotactic ablative radiotherapy (SABR).

You can find detailed information about stereotactic body radiotherapy in this section.


Proton therapy

This type of radiotherapy uses a different type of radiation beam called a proton beam. Protons collect energy as they slow down and travel through the body. They then release this energy at their target point – the tumour. This means they can give a higher dose of radiation straight to the cancer, but only to a very small area around it. So there is less chance of damage to nearby healthy tissue.

At the moment, proton beam radiotherapy machines in the UK are only able to treat cancer of the eye. They can't treat cancers deeper in the body. Some countries in Europe and the USA are testing and using proton beam radiation for deeper cancers, including a type of spinal cord tumour called chordoma. 

If proton treatment is suitable for you, the NHS Proton Overseas Programme pays to send you for treatment abroad, usually to the USA but sometimes to Switzerland. There are plans to make this type of proton treatment available in the UK from 2018. We have information about chordoma and proton beam treatment.


Electron beam radiotherapy

Electron beams cannot travel very far through body tissue. So doctors use this type of radiotherapy to treat skin cancers, or tumours very close to the surface of the body.

Sometimes electron beam therapy is used to give a booster radiation dose to the tumour or the area where the tumour was removed. For example, doctors sometimes give a booster dose at the end of a course of radiotherapy for breast cancer.


Adaptive radiotherapy

During a course of radiotherapy treatment a tumour often shrinks and changes shape. Adaptive radiotherapy means changing the radiotherapy plan after treatment has started. You may need it due to changes in the shape of the cancer or due to changes in your body shape, for example caused by weight loss. Your treatment team creates a new treatment plan and you may need to have some of your initial scans and X-rays repeated. 

Modern radiotherapy techniques can now give very precise adaptive radiotherapy. But this is quite a new technique and is not yet commonly used.


Effects of radiotherapy

External radiotherapy doesn't make you radioactive. It is safe to be with other people, including children, throughout your treatment.

People react to radiotherapy in different ways

  • Some carry on working and just take time off for their treatment
  • Others feel tired and want to stay at home
  • Occasionally some are admitted to hospital

Your doctors, radiographers and nurses will advise you about how best to manage your course of treatment. If you have family to look after you may need extra help. Don't be afraid to ask for help from your employer, family or friends, or the staff in the radiotherapy department. As your treatment goes on you'll have more idea of how it makes you feel. You can then make any necessary changes to your daily life to help you cope with the rest of your treatment course.

We have a whole section about radiotherapy side effects that you may find helpful.


More information about radiotherapy

We have detailed information about different types of radiotherapy in this section.

There is also detailed information about the side effects of radiotherapy.

We have information about how treatment teams plan external radiotherapy.

You can phone the Cancer Research UK nurses on freephone 0808 800 4040. The lines are open from 9am to 5pm, Monday to Friday. They will be happy to answer any questions that you have.

Our general organisations page gives details of people who can provide information about radiotherapy. Some organisations can put you in touch with a cancer support group. Our cancer and treatments reading list has information about books, leaflets and other resources about radiotherapy treatment.

If you want to find people to share experiences with online, you could use Cancer Chat, our online forum.

conformal radiotherapy

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Updated: 1 September 2014